WO 2010/064911 describes how to generate a layer of micro bubbles on the hull of a displacement vessel in order to reduce frictional drag, by means of a cavity being provided in a flat bottom area of the hull of a vessel, such that an opening of the cavity is at the flat bottom area. Air is injected into the cavity at such a rate that the water level in the cavity is kept substantially at the level of the hull of the vessel. Due to the forward movement of the vessel, the air in the cavity will move relative to the water at the speed at which the vessel is moving, or seen from the cavity, the water will flow past the cavity at that speed. This difference in velocity between the air and the water causes a so-called Kelvin Helmholtz Instability (KHI) which results in a mixing of air and water at the interface between the water and the air, and in a consequential generation of a layer of small sized air bubbles. The small size of these air bubbles makes them very stable and they tend to stay in the water for a relative long period. The small bubbles generated in the cavity are released there from at the rear edge of the cavity such that they form a stable lubricating layer extending a long distance along the bottom of the hull downstream of the cavity. In this way energy savings in propulsion of a vessel can be achieved.
It is an object of the invention to provide an improved system for providing an air lubricating layer between the hull of a vessel and the water flowing under the hull as the vessel is moving through the water, which can more easily be incorporated in the hull of a vessel.
It is a further object of the present invention to provide an improved system for providing an air lubricating layer that can be operated in an efficient and stable manner in wave conditions.
It is a further object of the invention to provide a displacement vessel incorporating such a system.
It is another object of the invention to provide a method of operating such a system in a displacement vessel.
It is another object of the invention to provide a method at retrofitting such a system into the hull of an existing displacement vessel, without weakening of the existing hull. It is another object to provide a method of retrofitting without interfering with the cargo space of the vessel.